The use of so-called "printing blankets" in offset lithography has been well known for a number of years. In the process of offset lithographic printing, a rotary cylinder is covered with a printing plate having a positive image area receptive to oil-based inks and repellent to water, as well as a background area in which the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with a rubber-surfaced ink receptive printing blanket. The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the blanket. Paper or other sheet stock to be printed is then passed between the blanket covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the blanket to the paper.
During the steps in which the image is transferred from the plate to the blanket and subsequently wherein the image is transferred from the printing blanket to the paper, it is important to ensure intimate contact between the two contacting surfaces. This is ordinarily achieved by positioning the blanket covered cylinder and the supporting cylinder it contacts so that there is a fixed interference between the two. Thus, the blanket is compressed throughout the printing run to a fixed depth, typically about 0.002 to 0.006 inches. Moreover, it is also important that this compression be maintained uniformly over the entire blanket.
This fixed interference between the blanket and the paper substrate may, for example, be accomplished by inserting one or more thin layers of paper or the like between the blanket and the surface of the cylinder to build up the thickness of the blanket. This process is known as "packing" a blanket.
Conventional offset printing blankets typically include a multi-ply fabric base and a vulcanized elastomeric face. Preferably, to enhance the compressibility of such blankets, one or more cellular compressible layer(s) may be buried within or attached to the fabric between the base and the elastomeric face of the blanket. These compressible layers may be formed with, for example, a plurality of either open (see, e.g., U.S. Pat. No. 4,442,895 to Shimura) or closed (U.S. Pat. No. 4,042,743 to Larsen) cell structures distributed within an elastomeric matrix.
Compressible layers of the type described above are particularly useful in preventing the occurrence of print distortion, i.e., a lack of definition caused by a buldge/deformation which often otherwise occurs in the printing surface of the blanket adjacent to the nip of the printing press. The compressible layer also serves to absorb the effect of a "smash", which is a substantial deformation of the blanket caused by a temporary increase in the thickness of the material to be printed, such as by the accidental introduction of more than one sheet of paper between the printing blanket and the back up cylinder during the printing operation.
Blankets of the type described above suffer from a variety of deficiencies, however, which negatively affect their durability and print quality. For example, they are susceptible to caliper loss due to compression set of one or more of the following: the fabric(s) used to form the support layer(s), the elastomeric compounds used to form the face and, where included, the adhesive layers for binding adjacent blanket segments, and/or, in the case of compressible blankets, the compressible matrix. In addition, printing blankets of the type described above also frequently suffer from wicking of ink, water and solvents commonly used in a press room, through either the exposed cut edges of the blankets or, in instances where these edges are protected by the application of a sealant, directly through cracks in the blanket or the bottom ply of the fabric.
Further, in blankets having a cellular compressible layer or "ply" (these terms are used interchangeably herein) the compressible ply is typically located below a single ply of fabric or else directly below the elastomeric printing face. This arrangement exposes the compressible layer to the high stress present at the adjacent printing face of the blanket, thus reducing the smash resistance of such blankets and increasing the occurrence of wicking through the cells therein.
Additionally, the application of the elastomeric printing face on prior art blankets is typically performed with the use of a technique known as knife over roll spreading, in which the face material is first dissolved in an organic solvent prior to its application upon the fabric ply or compressible layer. Such organic solvents, however, require sophisticated environmental controls to render them safe for use in the work place.
Furthermore, prior art printing blankets are often provided with surface profiles on their elastomeric printing face which fails to strike an acceptable balance between the print quality provided by this surface and its ability to release the printed stock in preparation for accepting the next impression from the printing cylinder.